1. Field of the Invention
This invention pertains to tanks or containers designed to contain liquids, underground, for dispensing. Specifically, a double walled tank, for the containment and dispensing of petroleum products, is addressed.
2. Background of the Prior Art
By far the dominant method of storing and dispensing liquids, particular petroleum products, from, e.g., service stations, is through the use of underground storage tanks. Conventionally, tanks, of generally cylindrical design, are prepared and installed in a pit, subsequently covered, said tanks having been provided with fittings, piping, valves and the like, such that gasoline or similar fuel products can be introduced to the interior of the tank, and then dispensed, under control. Initially, these tanks were prepared from steel. Steel continues to be a popular material from which to construct the storage tanks.
However, as discussed in U.S. Pat. Nos. 3,335,904 and 3,700,512 the problems associated with underground storage tanks, particularly posed by corrosion problems, and the crushing pressures applied, particularly if the hole should become filled with water, require for many situations the use of a superior material, such as fiber reinforced plastic, a particular example of which includes chopped fiberglass reinforced isophthalic resin. Note, in particular, the discussion at column 1, lines 64-72 and column 2, line 60--column 3, line 8 of U.S. Pat. No. 3,335,904 and particularly the discussion at column 2, lines 4-65 of U.S. Pat. No. 3,700,512.
In this respect, it should be noted that underground storage tanks, and factors addressed in their design, production, and maintenance, should be distinguished from containers intended for above-ground storage, such as for the storage and/or transportation of cryogenic materials. While both types of containers must have sufficient strength to contain the liquid in the interior, these cryogenic containers are generally constructed without regard to, or need to safeguard against, pressures and corrosive agents applied from the exterior, as in the case with underground storage tanks. U.S. Pat. Nos. 3,895,152 and 3,317,074 are exemplary references discussing cryogenic containers.
However, the use of fiber reinforced plastic materials for the construction of underground storage tanks has not totally eliminated the problem of environmental contamination or pollution. In particular, the tanks of the prior art, although exhibiting superior resistance to corrosive attack, nonetheless remain potentially subject to cracking and/or holing, particularly during handling, as well as after installation. For example, the rough treatment a tank may receive during transportation and manipulation prior to installation may result in undetected punctures of the tank. Similarly, when in operation, a shift in the bed may result in fracturing or holing of the tank, resulting in the spillage and resulting hazards addressed in U.S. Pat. No. 3,700,512.
Accordingly, many locations and authorities have instituted heightened standards for reliability for the construction and design of underground storage tanks, such as Calif. Even in regions where such stringent regulations are not enforced, avoidance of environmental pollution remains a concern of the highest priority.
One widely adopted response to this problem has been the provision of dual-walled storage tanks. An example of a double walled storage tank prepared from steel materials can be found in U.S. Pat. No. 1,886,074. However, as noted above, steel rusts. Double walled underground storage tanks have also been prepared out of fiber reinforced resin systems, such as that disclosed in U.S. Pat. No. 4,676,093. As described in both these references, the provision of two walls, alone, is insufficient. The use of a double walled storage tank is adopted to allow the installation of an alarm system, which will detect a leak in either the inner tank, the outer tank, or both. One such alarm system consists of detectors sensitive to the presence of fluid in the space between the inner and the outer tank. This is generally referred to as a "dry" alarm system. An alternative, or "wet" system, fills the space between the inner and the outer tank with a liquid, such as ethylene glycol, water or brine. When either the inner or the outer tank is punctured or otherwise develops a leak, at least a portion of the liquid contained between the two tanks will flow through the leak, the reduction in the volume of water held between the two walls being detected, by a variety of responsive means. Such systems again are described in U.S. Pat. No. 4,676,093, as well as U.S. Pat. No. 4,672,366.
However, the need to maintain a space between the inner and outer tanks of a double walled storage tank implies that two separate, strong tanks be constructed, and connected. In U.S. Pat. No. 4,676,093, as well as many other references, such as U.S. Pat. No. 4,739,659, this is achieved by the provision of ribs which extend between the first and second tank, providing contact therebetween, as well as circumferential reinforcement and deflection resistance. However, even in the design in U.S. Pat. No. 4,739,659, a certain amount of space between the inner and the outer wall must be left open, for the alarm or leak-detection system. This patent includes a "gas pervious" material between the inner and outer walls, the outer wall being discontinuous, tied into the side of each rib. The gas pervious material is not load-transferring, and strength is again provided by securing the outer tank wall to the ribs. Since both tanks have to be resistant to leakage or penetration, this implies that both tanks must have, independent, sufficient strength to resist puncture, fracture and compression, as they cannot share strength, no load being transmitted across the space between the two tanks, save for the limited case of the ribs. While the presence of the ribs may be employed to reduce deflection therebetween, spacing of the ribs does not permit a load or impact at a point between the ribs applied to, e.g., the outer tank, to be transmitted to, or shared with, the inner tank. Similarly, loads or impact supplied to the inner tank, cannot be shared by the outer tank. This results in independently strengthened inner and outer shells, and a significant consumption of material and man hours.
Accordingly, it is an object of this invention to provide a double walled tank, capable of being equipped with either a dry or wet alarm system, which comprises a inner and outer shell, between which load may be transmitted and shared, so as to reduce the need to provide two entirely independent tanks, and reduce the material, and man hours, involved in their construction.